Recycled　aggregate　(RA)　was　surface-treated　in　a　sulfoaluminate　cement　(SAC)　slurry　in　order　to　improve　the　resistance　of　recycled　aggregate　concrete　(RAC)　to　external　sulfate　attacks.　Three　groups　of　concrete,　were　prepared　with　non-treated　natural　or　recycled　aggregate,　and　surface-treated　RA　(SRA),　respectively.　The　results　indicate　that　the　mechanical　performance　of　SRAC　(recycled　concrete　prepared　with　SRA)　was　improved　compared　to　the　reference　RAC.　On　the　micro　scale,　a　relatively　dense　SAC　coating　was　observed　on　the　surface　of　SRA,　which　led　to　a　layered,　sandwich-shaped　structure　of　the　new　interfacial　transition　zone　(ITZ)　formed　between　SRA　and　the　new　cement　mortar　in　SRAC.　The　sandwich-shaped　new　ITZ　presented　an　overall　higher　micro-hardness　in　comparisons　with　that　in　the　reference　RAC.　When　subjected　to　external　sodium　sulfate　attacks　coupled　with　drying-wetting　cycles,　the　accumulation　of　sulfate　ions　was　slower　in　SRAC,　meanwhile　the　deterioration　in　mechanical　performance　of　SRAC　was　less　drastic　than　that　of　the　reference　RAC　after　a　long-term　sulfate　exposure.　This　paper　suggests　that　the　enhanced　sandwich-shaped　new　ITZ　should　help　hinder　the　SO42-　accumulation　in　SRAC.　Moreover,　the　dense　SAC　coating　which　well　sustained　the　sulfate-induced　tension　can　protect　the　old　ITZ　and　old　mortar　from　the　attacks　of　sulfate　ions.　This　explains　why　the　resistance　of　SRAC　to　external　sulfate　attacks　was　improved　by　the　SAC　treatment　on　RA.　©　2021